Air Carrier Captain reported taking off with less than expected runway length at GSO airport.

Narrative

When we loaded performance data; we used 'Runway 5R Shortened'. The available runway was 8;100 [ft]. Taxiing out; as we approached Kilo 1 Taxiway to join the Runway; a little voice in my head asked 'hey...was K1 available for data or was the data you loaded correct?' I was about to ask my FO (First Officer) what he recalled as options when I say the signs. At Kilo 1; there is a temporary sign that says Runway available 8;100 [ft]. The mistake I made was thinking that meant from that position; at Kilo 1. Tower never asked if I wanted to back taxi; and in hindsight there were a couple clues that K1 was not the end of the runway. However; I missed that and thought 'okay...8;100...that's what I remember seeing...we are good to go' The stop margin was 964 feet based on the shortened runway. However; we took off about 1;000 feet down from the real end of the runway; although I did cheat and grab a little extra runway with an opposite direction entry on to the runway. The net result of this is we took off with more or less zero stop margin. Takeoff was uneventful; although it was obvious as we rotated that we had less runway ahead of us that we are used to seeing. This was exacerbated by the fact that I've done most flying where longer runways are the norm.Cause: Looking back; there were several contributors to this error. First--I'm using ACARS computed data vice other system; which while safe is different than what I've used for 20 years. I'm not used to scanning for intersection options; etc on the ACARS screen when we do performance data; and when I saw the term 'shortened' I thought 'okay--they've got the data loaded for the current construction already built for us...' I basically saw that and didn't continue to look at the other options available on the screen. That error is on me. The second factor was signage. I actually thought 'should I back taxi here?' for a moment; but as I approached the runway I saw the signs (attached) that indicated 8;100 was available; and I specifically remember the 8;100 feet from our data. The problem was the proximity to the K1 sign had me interpret the sign as saying 'you have 8;100 from K1'. We didn't.